Variable expression in focal dermal hypoplasia. An example of differential X-chromosome inactivation.

We encountered three women from three generations of the same family with features of focal dermal hypoplasia (FDH). Two of the patients, the proposita and her mother, demonstrated severe manifestations, including skin, dental, skeletal, and visceral abnormalities. The proposita's grandmother, the first family member affected, had very mild expression, with aplasia cutis congenita and dental caries as the only features expressed. This family illustrates both the marked variability of expression and the proposed X-linked dominant mode of inheritance of FDH. We postulate that early embryologic random inactivation of the X chromosome bearing the mutant gene responsible for FDH is the cause of the variable expression.